FormerlyHemlock
Hero
I get all that (wow this feels like treading water). I never mentioned game balance, and I get simulationism. How about we stop talking about general simulation theory (which I understand just fine), and start talking about why you feel something is a problem if you think it will never come up - which is the only question I am asking.
Even in simulation situations, you don't need to figure out the ecology of a worm the PCs will never encounter, and nobody the PCs meet have ever encountered, and which has never had any impact on anything they will ever interact with. Your world is not perfectly realized, it cannot ever be perfectly realized, you can never have enough hours in your lifetime to work out how everything conceivable in your world could possibly work. So, and I hope we can agree on this, some things which the players will never interact with in any way will not be figured out by you in advance because they are not a priority for working out in your simulationist world. Agreed?
Assuming we agree, and since you previously said your players will never attempt something ridiculous like declaring their allies as hostile to game some benefit from a feat, that would mean that aspect of the feat never needs to be worked out by you - it's the worm. It's an minor aspect of the world that never needs to be worked out in advance, because it's not in advance of anything - nobody will care about the worm, or some silly interpretation of a feat.
Which comes down to the actual likely questions your players will ask - why does it only work on hostile creatures? So you can't think of a coherent reason why the feat only impacts people hostile to you? Is that really why you want to "fix" it, because you are having trouble coming up with a justification for why it only impacts hostiles? If so, it's likely a recurring problem, as it's the same issue inherent with opportunity attacks and several other mechanics in the game. I'd think it would be a lot easier, and more interesting, to come up with such a reason for your world rather than just change this mechanic to avoid inventing the reason on this rule (only to face the question again on other similar rules). Do you disagree?
Sigh. Yes, we're definitely treading water here, and I'm on phone so I can't go into detail. Your later paragraphs make it clear that you're aware of some issues that the rule creates, right now, today. So why do you lead by claiming that "it will never come up"?
I don't agree that the Warcaster issue would just recur with other mechanics like opportunity attacks. There's nothing that prevents you from attacking allies, it's just that no one does it in practice. That's nothing at all like being able to reflexively Polymorph people in the same round as casting other spells, if and only if the spell's target is an enemy retreating from melee with you. So yes, I disagree with your final paragraph and final question.